Can I swim in Lake Natoma? What to know about E. coli in Sacramento-area waterways
Is Lake Natoma safe to swim in this summer? State officials are not yet certain.
Ahead of the July 4 holiday weekend, California State Parks officials warned visitors to Lake Natoma that its water monitoring partner detected elevated levels of E. coli bacteria, indicating water pollution from fecal matter, The Sacramento Bee previously reported.
With some of the lake’s most popular recreation areas — Black Miners Bar Day Use Beach, Nimbus Flats and Willow Creek Day Use Area — deemed a health concern, some lake visitors might be second-guessing their trips and looking for alternative places to cool off in the water.
Here’s what state health officials recommend for capital region swimmers wanting to hit the lake this summer.
Is it safe to swim in water with E. coli?
Generally, many strains of E. coli are relatively harmless to humans, according to the Mayo Clinic, but some strains can lead to uncomfortable gastrointestinal symptoms.
Loose stool, stomach cramping or pain, upset stomach and vomiting can occur typically three to four days after initial contact with the E. coli bacteria, Mayo Clinic said. Though most cases of infection pass, some strains can lead to a life-threatening type of kidney failure, particularly in young children and older adults.
The risk is higher for pets, in particular young puppies and immunocompromised animals, according to PetMD. Similar gastrointestinal symptoms to human E. coli infection can be present in pets, but more dangerous outcomes are possible as well. Pneumonia, urinary tract infections and even sepsis can occur in pets who have been infected by E. coli, the veterinary blog wrote.
Because of this, California State Parks urges visitors “out of an abundance of caution” to avoid the parts of Lake Natoma where elevated levels of the bacteria were detected.
“While most E. coli strains are harmless and naturally present at low levels in many waterbodies, it is important to exercise caution when concentrations are elevated,” the state agency wrote in an emailed statement.
How long is Lake Natoma water unsafe to swim in?
The Central Valley Regional Water Quality Control Board will continue testing the waters in and around Lake Natoma to determine when levels are no longer elevated, in partnership with California State Parks.
In its statement, the state parks agency said that cautionary measures will remain in place until two consecutive weeks of testing show acceptable levels of E. coli in the water.
“Because water quality conditions can change, we are unable to predict how long elevated E. coli levels will persist,” the State Parks statement read. “Ongoing monitoring will determine when conditions have improved and it is appropriate to remove the advisories.”
As of Wednesday, the Willow Creek Day Use Area had three measurements below the threshold value of bacteria concentration since it first measured elevated on June 16. The Nimbus Flat North Beach area had two measurements within acceptable levels after its water first tested above the threshold on June 23.
As for the Black Miners Bar Swim Area, its water has had high concentration of E. coli for six out of the last eight measurements, per data from the water board’s Surface Water Ambient Monitoring Program. The program’s June 16 testing showed Black Miners Bar’s water jumped by more than 18 times its prior week concentration.
Where to swim around Sacramento to avoid E. coli
Only the parts of Lake Natoma where tested water detected high E. coli levels are considered unsafe for visitors, California State Parks wrote.
All testing points of Folsom Lake, just upstream from Lake Natoma, were within acceptable concentration levels of the bacteria as of Tuesday, according to the Central Valley Water Board. Other areas of Lake Natoma, including the Black Miners Bar Boat Ramp and Nimbus Flat South Beach, were also below the threshold point Tuesday.
The Surface Water Ambient Monitoring Program considers a measurement of 320 most probable number per 100 milliliters — a common unit of measurement for estimating bacteria count — as the upper limit of acceptable E. coli concentration when testing Lake Natoma water.
The Lower American River, which is fed by Lake Natoma and Folsom Lake, is no longer monitored by the water board for E. coli as of June 2025.
In Lake Tahoe, there are no active E. coli warnings or alerts, according to the U.S. Forest Service’s website. In 2025, the lake’s southern Camp Richardson Resort measured elevated E. coli levels near the shore for roughly two weeks in August before returning to normal, according to the Lake Tahoe Basin Management Unit.
How to stay safe from E. coli exposure in the water
The best way to prevent exposure from waterborne E. coli is to not get in contaminated water at all. However, it can be difficult to be certain whether the water has concerning levels of the bacteria without consistent testing.
For people swimming in the Sacramento region’s natural waterways, California State Parks recommends not drinking or swallowing lake or river water and avoiding swimming with open wounds. After getting out of the water, swimmers should shower with clean water soon after, according to the agency.
Pet owners are encouraged to keep their furry friends away from areas under the health advisory and watch them closely for any symptoms of bacterial infection after playing in lake or river water.
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